I. Field of the Invention
The present invention relates to data communications. More particularly, the present invention relates to a novel and improved method and apparatus for encoding and decoding trellis modulated data based upon punctured convolutional codes.
II. Description of the Related Art
The field of data communications is concerned with raising the data throughput of a transmission system with a limited signal to noise ratio (SNR). The use of error correcting circuitry such as the Viterbi decoder allows system tradeoffs to be made with smaller SNRs or higher data rates to be used with the same bit error rate (BER). The decrease in the SNR needed is generally referred to as coding gain. Coding gain may be determined from bit error performance curves. In a graph of bit error performance curves, the BER of uncoded and various coded data is charted against Eb/No, where Eb is the energy per bit and No is the one sided Gaussian White Noise power spectral density. The coding gain at any point along a bit error performance curve for a particular BER level is determined by subtracting the coded Eb/No from the uncoded Eb/No. In the paper xe2x80x9cViterbi Decoding for Satellite and Space Communicationxe2x80x9d, by J. A. Heller and I. M. Jacobs, IEEE Transactions on Communication Technology, Vol. COM-19, pgs. 835-848, October 1971, extensive results of simulations on various decoder apparatus were reported.
The coding rate and constraint length are used to define the Viterbi decoder. The coding rate (m/n) corresponds to the number of coding symbols produced (n) for a given number of input bit (m). The coding rate of xc2xd has become one of the most popular rates, although other code rates are also generally used. One class of codes with mxe2x89xa01 are called punctured codes and are produced by discarding or erasing symbols from the rate 1/n code. The constraint length (K) is related to the length of the conventional encoder used in the encoding of the data. A constraint length of K=7 is typical in convolutional coding schemes. The convolutional encoder can be thought of as a Finite Impulse Response (FIR) filter with binary coefficients and length Kxe2x88x921. This filter produces a symbol stream with 2Kxe2x88x921 possible states.
The basic principal of the Viterbi algorithm is to take a convolutionally encoded data stream that has been transmitted over a noisy channel and use the properties of the convolutional code to determine the transmitted bit stream. The Viterbi algorithm is a computationally efficient method of updating the conditional probabilities of all 2Kxe2x88x921 states and finding the most probable bit sequence transmitted. In order to compute this probability, all the conditional probabilities of 2Kxe2x88x921 states for each bit must be computed. For a rate xc2xd code, the resulting decision from each of these computations is stored as a single bit in a path memory.
A chainback operation, an inverse of the encoding operation, is performed in which the pxc2x72Kxe2x88x921 decision bits are used to select an output bit, where p is the path memory depth. After many states the most probable path will be selected with a high degree of certainty. The path memory depth must be sufficiently long to allow this probability to approach 1. For a rate xc2xd code, an exemplary path memory depth is about (5xc2x7K), or 35 states. For a rate xe2x85x9e punctured code the optimal depth increases to 96 states.
Constraint lengths of K less than 5 are too small to provide any substantial coding gain, while systems with K greater than 7 are typically too complex to implement as a parallel architecture on a single VLSI device. As the constraint length increases, the number of interconnections in a fully parallel computation section increases as a function of (2Kxe2x88x921xc2x7L), where L is the number of bits of precision in the state metric computations. Therefore, where K is greater than 7, serial computation devices are generally used which employ large external random access memories (RAMs).
In the paper xe2x80x9cChannel Coding with Multilevel/Phase Signalxe2x80x9d by G. Ungerboeck, IEEE Transactions on Information Theory, Vol. IT-28, pgs. 55-67, January 1982, a trellis coded modulation (TCM) was described. In Ungerboeck it was shown that within a given spectral bandwidth, it is possible to achieve an Asymptotic Coding Gain of up to 6 dB by employing a rate (nxe2x88x921)/n convolutional code and doubling the signal set. Unfortunately, for each modulation technique and for each bit rate, the maximal coding again is achieved by a different convolutional code. Further disclosed were the results of a search for all convolutional codes for several rates and modulation techniques, and the best codes presented.
In the paper xe2x80x9cA Pragmatic Approach to Trellis-Coded Modulationxe2x80x9d by A. J. Viterbi, J. K. Wolf, E. Zehavi and R. Padovani, IEEE Communications Magazine, pgs. 11-19, July 1989, a pragmatic approach to trellis coded modulation (PTCM) was disclosed. The underlying concept therein is that a somewhat lower coding gain is achievable by a PTCM based on the xe2x80x9cindustry standardxe2x80x9d rate xc2xd, K=7 convolutional code. Although a lower coding gain is realized, it is very close to the coding gain of Ungerboeck at BERs of interest.
Trellis coding is an attractive coding technique since it possesses an aspect which other coding techniques lack. The power of trellis coding lies in the fact that even though no apparent coding operation is performed on some of the bit(s) of the input data, the decoder is able to provide error correction on all bits. Generally the use of TCM techniques to achieve efficient use of power-bandwidth resources has been limited to low speed applications in digital signal processor implementations. The use of PTCM techniques enable VLSI implementations of an encoder/decoder capable of operating at high rates. A decoder using PTCM techniques is capable of handling different modulation techniques, such as M-ary phase-shift keying (M-ary PSK) including Binary PSK (BPSK), Quadrature PSK (QPSK), 8-PSK, and 16-PSK.
In the paper, xe2x80x9cDevelopment of Variable-Rate Viterbi Decoder and Its Performance Characteristics,xe2x80x9d Sixth International Conference on Digital Satellite Communications, Phoenix Ariz., September 1983, Y. Yasuda, Y. Hirata, K. Nakamura and S. Otani discuss a method whereby a class of high rate binary convolutional codes can be constructed from a single lower rate binary convolutional code. The advantage of punctured codes for binary transmission is that the encoders and decoders for the entire class of codes can be constructed easily by modifying the single encoder and decoder for the rate xc2xd binary convolutional code from which the high rate punctured code was derived. The current invention will be concerned primarily with rate (mxe2x88x921)/m binary convolutional codes (m a positive integer greater than or equal to 3) formed from puncturing a particular rate xc2xd convolutional code which has become a de-facto standard of the communications industry. This code has constraint length 7 and generator polynomials G1((D)=1+D2+D3+D5+D6 and G2(D)=1+D+D2+D3+D6. Indeed, many commercial VLSI convolutional encoder and decoder chips (including a device marketed under Part No. Q1875 by QUALCOMM Incorporated of San Diego, Calif.) contain encoders and decoders for punctured binary codes using this de-facto standard rate xc2xd code.
It is therefore an object of the present invention to provide a novel method and circuitry for encoding and decoding trellis data using punctured rate xc2xd convolutional encoders.
The present invention is a novel and improved method and apparatus for encoding and decoding trellis modulated data based upon punctured rate xc2xd convolutional codes. In accordance with the present invention, a trellis encoder and decoder are disclosed in which a circuit is provided that encodes and decodes based upon punctured rate xc2xd convolutional encoding.
In the present invention, a transmitter encodes a bit stream, parallel or serial, according to a k/n encoding format provided by a punctured rate xc2xd encoder. For each set of k input bits n output symbols are provided by the first encoder. The n output symbols are then provided to a parallel n to parallel p converter that in effect gates the data symbols to provide sets of data of p symbols each. If n is divisible by p, the converter acts as a buffer to provide p symbols in each set. Otherwise the converter will wait for the next group of n symbols before providing the last group of p symbols from the current n symbols.
Each set of symbols is provided to an interleaver comprised of pxe2x88x921 parallel delay elements that can be implemented as digital delays or delay lines having delays of predetermined duration. The purpose of the interleaving proces is to increase the diversity in the data in such a way that if a modulated symbol is lost in transmission the error may be recoverable with the use of error correcting algorithms in the decoder. The output of the delays and the one undelayed symbol are then provided to an m-ary modulator that provides a modulation in accordance to the received bits.
In the receiver, a decoder presents the received signal to a demodulator that converts the modulation values to sets of p symbols. These sets of symbols are then provided to a deinterleaver that has pxe2x88x921 delay elements that can be implemented as digital delays or delay lines. The three deinterleaved symbols are provided to three separate metric calculators that calculate from the received symbols metrics that define quantities proportional to the likelihood of alternative received hypotheses. The metrics are provided to a parallel p to parallel n converter that provides the metric values of data as n metrics to a rate xc2xd punctured decoder that operates with the aid of the Viterbi algorithm to provide a corrected estimate of the transmitted bit sequence.